This invention relates to radio antenna systems and methods, particularly antenna systems and methods for enabling one antenna to transmit a high frequency radio signal and simultaneously receive a low frequency radio signal.
In vehicles, such as automobiles or airplanes, it is often desirable or necessary to employ both radio navigation and radio communications equipment. A common type of radio navigation is a system known as LORAN, which works by receiving at a vehicle encoded low frequency radio signals transmitted from fixed locations, based upon which the vehicle's position can be computed. LORAN-C is the LORAN system currently in predominate use, and its signals are broadcast at a carrier frequency of 100 kilohertz ("kHz").
In contrast, two-way radio communications with land and air vehicles typically employ VHF frequencies in the VHF band. VHF carrier frequencies range from about 30 megahertz ("MHz") up to about 300 MHz. Typically, a vehicle would carry a VHF radio frequency transceiver. It is often desirable for the vehicle operator simultaneously to be receiving LORAN signals for navigation and to be transmitting or receiving VHF signals for communication.
Notwithstanding the considerable difference in frequency of LORAN and VHF signals, it is often desirable to employ the same antenna on a vehicle both to receive low frequency LORAN signals and to transmit or receive VHF communication signals. Moreover, it is ordinarily desirable that the antenna be relatively short, on the order of about 20 inches. In the case of an automobile one reason is to minimize the obtrusiveness of the antenna. This may be because the user does not want an unsightly antenna on the car, because some degree of concealment of the antenna may be important to law enforcement agencies, or simply to avoid attracting the attention of persons who may be tempted to steal expensive electronic equipment within the vehicle. In the case of an airplane, the use of a single, short antenna contributes to the operation of the airplane by minimizing the number and size of protrusions from the airplane so as to decrease drag. It is also important to minimize the number of items that would otherwise contribute unnecessary weight to the aircraft. Thence, it is desirable to use a single, relatively short antenna both to receive low frequency navigation signals, such as LORAN, and simultaneously transmit or receive high frequency, typically VHF, radio communications.
There are some serious obstacles to achieving the foregoing objective. In many vehicles, particularly in aircraft, the antenna must be located some distance from the LORAN receiver and from a VHF transceiver. For example, the antenna would typically be located on the top of an airplane fuselage, while the LORAN navigation equipment and VHF transceiver would be located in the cockpit. Ordinarily, this would require two separate transmission lines between the antenna and the radio electronics, which is not only inconvenient but contributes undesired weight to the aircraft. Another significant problem is that, while VHF signals have wavelengths not too much greater than such a short antenna as a result of which the antenna has a relatively low impedance at VHF frequencies, the wavelength of LORAN signals is much longer than the antenna, so that the antenna has a very high impedance at low, LORAN signal frequencies. This means that it is relatively easy to match the antenna to the VHF transceiver for maximum transfer of power, but that it is difficult to do so for a LORAN signal. Consequently, use of a short antenna to receive LORAN signals will ordinarily result in a low signal-to-noise ratio.
Use of a single antenna both for transmitting VHF communications and receiving LORAN must also be accomplished such that the transmitted VHF signal does not damage, or even overload, the input of the LORAN receiver. Also, such an antenna should be versatile, so that it can be used with a variety of electronic equipment.
A number of systems for employing the same antenna to both receive relatively low frequency radio signals and to transmit or receive relatively high frequency radio signals have been developed. Such systems are disclosed, for example, in Tanner, et al. U.S. Pat. No. 4,268,805; Elliott, U.S. Pat. No. 4,095,229; and Tyrey, U.S. Pat. No. 4,037,177. However, in each of these either the radio equipment would have to be located relatively close to the antenna, or more than one transmission line would be required to connect low and high frequency matching networks to the radio equipment. Moreover, none of these systems deals with the simultaneous use of LORAN reception and VHF transmission. Powell, et al. U.S. Pat. No. 3,812,494 discloses a system for receiving and transmitting Doppler frequencies and telemetering frequencies simultaneously over the same antenna, as distinguished from low frequency LORAN and VHF frequencies, and employs an impedance matching network in close proximity with the antenna itself. Duncan, Jr., et al. U.S. Pat. No. 3,217,273 discloses a system for coupling several transmitters and receivers to a single antenna, while avoiding the radiation of spurious signals resulting from intermodulation, and avoiding swamping the receivers. In addition, LORAN antenna systems which employ a current regulated preamplifier located at a remote antenna, and powered by DC current provided over the antenna-to-receiver transmission line, to prevent reduction of the signal-to-noise ratio between the antenna and the LORAN receiver have previously been used. However, none of these systems solves the aforementioned problems associated with employing a relatively short antenna disposed some distance away from LORAN and VHF communications equipment both to receive LORAN signals and transmit or receive VHF communications simultaneously.